The present invention relates to a method for producing eutectic ceramics. More particularly, the present invention relates to a method for producing a eutectic crystal structure (hereinafter simply called xe2x80x9ceutecticxe2x80x9d) from eutectic powder of a rare earth aluminate and alumina or a rare earth oxide. Specifically, the present invention provides dense sintered eutectic ceramics compact which are almost indestructible in comparison with conventional materials, and can be used in various types of industrial applications.
Conventionally, eutectic ceramics such as a eutectic of alumina and a rare earth aluminate compound are produced using the Bridgman technique, which is one type of single crystal growth technique. Specifically, a formed product with a desired shape is produced by unidirectional solidification which consists of filling a crucible made of molybdenum or tungsten with a specimen, melting the specimen at a high temperature, and slowly cooling the specimen from the bottom, thereby allowing a eutectic crystal to be grown continuously from the bottom to the top (D. Viechnicki and F. Schmid, J. Mater. Sci., 4 (1969) 84-88). This eutectic is an excellent high-temperature material maintaining strength up to 1700xc2x0 C. (Y. Waku, H. Ohtsubo, N. Nakagawa, and Y. Kohtoku, J. Mater. Sci., 31 (1996) 4663-4670).
In the above method for producing eutectics using unidirectional solidification, the size of the specimen is limited depending on the size of the crucible. Moreover, since only a cylindrical eutectic is produced, it is necessary to process the eutectic into a specific shape suitable for actual application. Furthermore, synthesis of the specimen using a conventional method requires a slow cooling step, whereby a long period of time is needed to allow a large eutectic to be grown.
In principle, the above eutectic structure is obtained by melting the raw materials for the eutectic composition and causing the melt to solidify. For example, the eutectic is obtained in a short period of time by filling a water-cooled copper container with a specimen and dissolving the specimen using an arc or electron beams. However, since this method cannot allow the specimen to be homogenously dissolved, the resulting eutectic structure becomes inhomogenous and includes a large number of pores, whereby a large number of cracks is produced. Because of this, it is difficult to produce a large eutectic material.
A eutectic with a comparatively homogenous structure can be obtained by placing the raw materials for the eutectic composition in a large furnace and dissolving the raw materials only at the center portion using an arc. In this case, cracks also tend to be produced, whereby it is difficult to obtain a large eutectic.
Eutectic powder is obtained by grinding the eutectic obtained using these methods. A eutectic with a desired shape can be obtained by subjecting the eutectic powder to crystal growth or sintering. However, conventional sintering techniques do not allow the eutectic powder to be sufficiently grown to form a large eutectic.
Conventional sintering techniques used herein include a pressureless sintering method, a hot press method, and a hot isostatic pressing (HIP) method. In the pressureless sintering method, a starting powder material is sintered only by heating. The hot press method and the HIP method apply pressure in addition to heat in order to promote sintering. These two methods have an advantage in comparison with the pressureless sintering method in that the powder material can be sintered at a lower temperature. The difference between the hot press method and the HIP method is the capability of increasing the pressure to be applied.
These three methods promote the diffusion of substances using heat or heat and pressure in combination, thereby sintering the substances. However, a eutectic has a structure in which two constituent single crystals are intermingled. Therefore, these two single crystals must be separately grown in order to obtain a large eutectic using the eutectic powder as the raw material. This is a phenomenon differing from sintering for conventional polycrystals. Therefore, each crystal in the eutectic powder cannot be individually bonded by conventional sintering methods. As a result, only areas in which the same crystals happen to be located side by side are bonded, whereby a large number of pores is allowed to remain. This hinders densification of a sintered body.
An object of the present invention is to provide a method for advantageously producing eutectic ceramics having a homogenous and dense structure, in particular, a eutectic containing a rare earth aluminate compound.
In order to achieve the above object, the present invention essentially provides the following production methods. Specifically, the present invention provides a method for producing eutectic ceramics comprising allowing eutectic ceramics powder to stand at a temperature of 500-2000xc2x0 C. for 1-120 minutes under vacuum or in an non-oxidative atmosphere under a pressure of 5-100 MPa using a spark plasma sintering process, thereby causing crystal growth to occur.
In particular, the present invention provides a method for producing a eutectic containing a rare earth aluminate compound, comprising allowing eutectic powder of alumina and rare earth aluminate compound to stand at a temperature of 1300-1700xc2x0 C. for 1-120 minutes under vacuum or in an non-oxidative atmosphere under a pressure of 5-100 MPa using a spark plasma sintering apparatus, thereby causing crystal growth to occur to obtain a sintered body of a rare earth aluminate eutectic structure.
In the present invention, the eutectic powder used as the starting raw material is preferably one eutectic powder of alumina and a rare earth aluminate compound selected from the group consisting of Al2O3 and Ln3Al5O12, Al2O3 and LnAlO3, Ln2O3 and Ln2Al5O12, Ln2O3 and LnAlO3, and Ln2O3 and Ln4Al2O9, or one ceramic powder selected from the group consisting of MgAl2O4xe2x80x94LnAlO3, MgOxe2x80x94Al2O3, MgOxe2x80x94CaO, Al2O3xe2x80x94Nb2O5, CaOxe2x80x94Al2O3, Al2O3xe2x80x94ZrO2, B4Cxe2x80x94SiC, B4Cxe2x80x94TiB2, B4Cxe2x80x94YB6, PbOxe2x80x94Fe2O3, PbOxe2x80x94Nb2O5, PbOxe2x80x94V2O5, PbOxe2x80x94GeO2, BaOxe2x80x94WO3, V2O5xe2x80x94BaO, Bi2O3xe2x80x94GeO2, V2O5xe2x80x94ZnO, PbOxe2x80x94WO3, PbOxe2x80x94ZnO, Bi2O3xe2x80x94Fe2O3, V2O5xe2x80x94Cr2O3, Li2WO4xe2x80x94WO3, V2O5xe2x80x94MnO, V2O5xe2x80x94NiO, V2O5xe2x80x94CuO, Bi2O3xe2x80x94Al2O3, V2O5xe2x80x94CaO, Bi2O3xe2x80x94Mn2O3, Bi2O3xe2x80x94TiO2, CaOxe2x80x94WO3, SrOxe2x80x94WO3, MgOxe2x80x94WO3, Fe2O3xe2x80x94Ln2O3, and Nb2O3xe2x80x94Bi2O3.
Among the eutectic ceramicss obtained by the present invention, examples of the rare earth alumunate eutectics include an Al2O3xe2x80x94Ln3Al5O12 eutectic, Al2O3xe2x80x94LnAlO3 eutectic, Ln2O3xe2x80x94Ln3Al5O12 eutectic, Ln2O3xe2x80x94LnAlO3 eutectic, Ln2O3xe2x80x94Ln4Al2O9 eutectic, MgAl2O4xe2x80x94LnAlO3 eutectic, MgOxe2x80x94Al2O3 eutectic, MgOxe2x80x94CaO eutectic, Al2O3xe2x80x94Nb2O5 eutectic, CaOxe2x80x94Al2O3 eutectic, Al2O3xe2x80x94ZrO2 eutectic, and the like.